1. Field of the Invention
The present invention relates to a manufacturing method of a semiconductor device comprising an STI (Shallow Trench Isolation) edge, and more particularly, to a method of oxidizing a trench formed in a semiconductor substrate.
2. Description of the Related Art
Heretofore, in a semiconductor device such as a DRAM, an STI technique has been used to form, on one surface of a silicon substrate, element regions surrounded by a trench filled with an insulator, and MOS transistors having source and drain regions are formed in the element regions, and then gate electrodes, capacitors and wire layers are formed in these element regions. When such a semiconductor device is manufactured, an STI region is formed by filling the insulator into the trench formed in the silicon substrate before an insulating film remaining on the silicon substrate in the vicinity of the trench is removed, and a gate oxide film and the gate electrode are formed on the semiconductor substrate in the vicinity of the trench. Such a semiconductor device has a configuration having a pattern in which the gate electrode extends from the region adjacent onto the oxide film filled into the trench.
The semiconductor device described above has a configuration in which an edge of the STI region is adjacent to the gate insulating film and the gate electrode. If TZDB (Time Zero Dielectric Breakdown) characteristics of the semiconductor device with such a configuration is evaluated, it is found that the dielectric breakdown is often caused in the gate insulating film having a thickness of 6 nm by a voltage of 8 V or less (i.e., an electric field intensity of 13.3 MV/cm or less). On the other hand, it is presumed that the dielectric breakdown is caused by the STI edge since the dielectric breakdown having the TZDB characteristics is not caused in a pattern without the STI edge even by application of the voltage mentioned above.
On this presumption, the STI edge is carefully observed, and it is observed that a portion which forms the edge of the trench is thinner than other portions in the insulating film remaining in the trench.
Thus, a method of forming the trench and the insulating film has been reviewed. In this case, the trench is formed by etching the silicon substrate through STI dry etching, and the insulator buried in the trench is formed by oxidizing the inside of the-trench with water produced from the combustion of hydrogen and oxygen and by a CVD oxide film, after the etching and cleaning processes mentioned above.
On the other hand, Japanese Patent Publication Laid-open No. 2000-269499 (hereinafter referred to as a reference 1) points out that the gate oxide film deposited at a trench upper corner of a P-channel power MOSFET has a thickness smaller than those in flat portions or on a trench sidewall. Furthermore, it is also pointed out that the electric field concentrates on the thin oxide film at the corner portion to cause the dielectric breakdown of the gate insulating film.
In order to prevent such a dielectric breakdown of the gate insulating film, the reference 1 proposes a MOSFET in which the gate insulating film located at the corner portion of the trench has a thickness greater than the thickness of other portions of the trench. The reference 1 also discloses that an impurity concentration in the silicon substrate region adjacent to the corner portion of the trench is locally made higher than the impurity concentration in other portions so as to form the gate insulating film with greater thickness at the corner portion. It points out that if, in this way, RIE (Reactive Ion Etching) is utilized to etch the region containing a high concentration of impurities, and a trench is formed, and then a gate oxide film is formed in the trench by a hydrochloric acid dilution oxidization method, an oxide film having nearly double the thickness is formed in the region containing the high concentration of impurities (paragraph 0031).
Next, Japanese Patent Publication Laid-open No. 6-267938 (hereinafter referred to as a reference 2) describes a method of forming an oxide film wherein 2 to 8 wt % of trans-1,2-dichloroethylene is added in an oxidizing atmosphere, in order to form a silicon oxide film with high quality on the silicon substrate. It is shown that, in accordance with this method, impurities such as heavy metals and alkali ions can be gettered by adding trans-1,2-dichloroethylene.
Furthermore, Japanese Patent Publication Laid-open No. 63-316440 (hereinafter referred to as a reference 3) discloses that when a trench is formed in the silicon substrate by the reactive ion etching, a mixed gas of chlorine and oxygen can be used as an etching gas to prevent a rough bottom, a rough sidewall and the like of the trench that are caused by precipitation of carbon in the etching with a gas containing carbon such as carbon tetrachloride.
Still further, Japanese Patent Publication Laid-open No. 11-274288 (hereinafter referred to as a reference 4) describes that an edge of an active region is rounded (paragraph 0006) so as to prevent the electric field concentration on the corner produced in the active region adjacent to the trench. Further, the reference 4 proposes rounding the corner portion of the active region to eliminate acute angle shaped portions in the active region and an oxide film region adjacent to the trench, as well as performing thermal oxidation to provide climbing to a polycrystal silicon film side. In this case, in the trench and the active region, a silicon oxide film having a protrusion protruding like a bird's beak is formed, and the corner portion of the active region is positioned under the protrusion, and a roundly bored concave portion which retreats inward from the corner portion on an upper side thereof is formed (paragraph 0022). The reference 4 also discloses that dry oxidation at 1100° C. and HCl oxidation at 1000° C. or higher, for example, 1100° C. can be used as a method of forming the silicon oxide film shaped like the bird's beak in the trench and active region (paragraph 0023).
Reviewing further here the references 1 to 4, the reference 1 only discloses a method of manufacturing the P-channel power MOSFET, and does not give any consideration of the MOS transistor having a shallow STI region. That is, the reference 1 makes it clear that in the MOSFET comprising a deep trench, a high concentration impurity region is formed only in the region close to the corner portion of the trench, and then a thick gate oxide film is formed on the high concentration impurity region by the hydrochloric acid dilution oxidization method, thereby enabling the thick oxide film to be formed at the corner portion of the trench. In other words, the reference 1 discloses a method of providing a thick oxide film at the corner portion by the combination of the impurity concentration at the trench corner portion and the hydrochloric acid dilution oxidization method.
As apparent from this, the reference 1 does not at all suggest a method of forming an oxide film having a shallow trench such that a high concentration impurity region can not be formed, and a method of forming a locally thick oxide film without using the region in which the impurity concentration is high.
Furthermore, the reference 2 only discloses the method of forming the oxide film using trans-1,2-dichloroethylene, and does not at all indicate a method of locally thickening only part of the oxide film when a trench is formed. In addition, the reference 2 proposes setting the content of dichloroethylene in the oxidizing atmosphere to 2.0 to 14 wt % to remove a natural oxide film having a thickness of 0.5 to 1.4 nm and to form a silicon oxide film with high quality, but does not give any consideration of the oxide film formed inside the trench.
Still further, the reference 3 discloses a method of forming a trench through etching with chlorine and oxygen gases instead of a gas containing carbon so as to eliminate a disadvantage caused when the trench is formed through etching with the gas containing carbon. During this etching, a deposit containing silicon dioxide as a main component is deposited on the trench sidewall, and this deposit is removed by fluorine (page 3, upper right column and lower right column). However, the reference 3 does not refer to a treatment after the trench is formed with chlorine and oxygen gases and the deposit is removed. Especially, the reference 3 proposes formation of the oxide film with chlorine and oxygen gases instead of the gas containing carbon, and does not at all disclose a solution when carbon is contained as in dichloroethylene (DCE), for example.
Next, the reference 4 proposes rounding the corner portion at an upper position in the element region by oxidation, and embedding into the trench a silicon oxide film protruding further than an element region surface by the thermal oxidation. However, the reference 4 does not indicate a method of forming an oxide film having locally different thickness in a single oxide film formation, and a change in the thickness of the oxide film inside the STI region, and it does not at all describe conditions required for the oxide film formation, such as the concentration of HCl oxidation.